This invention relates to permanent magnet machines and, more particularly, to a permanent magnet machine having a unique fractional stator slot/rotor pole arrangement and a novel coil winding configuration.
The use of permanent magnet machines is abundant. A permanent magnet machine generally consists of a rotor that includes an even number of equally spaced magnets which have an alternating polarity. The magnets are placed around the inner radial surface of the rotor having a central shaft and a stack of laminations that are mounted on the shaft. The stack of laminations thereby hold a plurality of permanent magnets for producing an even number of magnetic poles on the periphery of the rotor. The conventional permanent magnet machine also includes a stator having a primary winding for generating a rotating magnetic field to provide rotation torque under energization by AC power. When a current is provided to the coils, the teeth of the stator become polarized and the permanent magnets in the rotor rotate due to the magnetic flux. In constant operation, the rotor is locked into particular synchronous speeds.
Electric motors with permanent magnet rotors often suffer from a condition called "cogging." Cogging is a variation in motor torque caused by variations in magnetic flux due to the alignment of the rotor and the stator teeth at various positions of the rotor. The cogging torque is superimposed on the driving torque developed by the magnetic flux caused by the fundamental component that causes the rotor to turn, but the cogging torque does not aid the driving torque. Rather, the cogging torque serves to negatively oscillate the rotor, and these oscillations are further transferred to the motor frame or equipment driven by the motor, thereby producing undesirable oscillation and noise. Cogging can therefore reduce the efficiency and the reliability of the motor.
Various methods have been implemented in the past to the cogging problem. One such method known to reduce cogging is to skew the permanent magnets in the rotor in either an angled or a herringbone pattern. Skewing the magnets in these ways, however, creates additional difficulties in that it is difficult to skew the position of a magnet in a rotor, and a skewed design further requires additional elements for effective operation, such as a sensor.
Another method used to reduce cogging is implementing a fractional stator slot/rotor pole combination. Such a combination has been commonly used to reduce cogging. An example is with disk drives, wherein the permanent magnet motor has stator slot/rotor pole ratio of 9/8. Such a combination is commonly used for high speed machines, but this stator slot/rotor pole arrangement commonly has a noise problem and it is often difficult to get the poles balanced.
Therefore, what is needed and not found in the prior art is a brushless permanent magnet machine that reduces the problem of cogging and that is easy to assemble and operate.